[jhasse]

Also happens on startup for me and others. See for example this comment from the bug report:

I simply double-clicked the Firefox icon twice quicker than I ever normally would, and the Close Firefox error appears: "Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system."

[digi_owl]

Indeed. This in particular when launching Firefox in response to opening urls from external sources.

Hit two of those quickly and hello dialog...